well it can either be 5, 6, 7, or 8. so how can u be sure its 5? although this information below does seem to support the gobi (5) or the hachibi (

being the uncaptured jinchuuriki
according to a website:
Unknown Jinchuuriki One - Akatsuki captured this Jinchuuriki and its Bijuu at some point in the past. It is unknown what tailed demon it was and which Akatsuki member captured it. The only firm details are that the Jinchuuriki was an outcast, who was abandoned by their village when Akatsuki came to capture them.
Unknown Jinchuuriki Two - Akatsuki captured this Jinchuuriki and its Bijuu at some point in the past. It is unknown what tailed demon it was and which Akatsuki member captured it. The only firm details are that the Jinchuuriki was an outcast, who was abandoned by their village when Akatsuki came to capture them.
Unknown Jinchuuriki Three - Nibi was the sixth bijuu sealed. At some point after sealing away Ichibi and before Sanbi, another bijuu was sealed away. It is unknown what tailed demon it was and which Akatsuki member captured it. It is also unknown whether it was a Jinchuuriki or a wild bijuu. However, with Tobi's surprise concerning the wild bijuu Sanbi, this third unknown bijuu was probably in a Jinchuuriki.
Unknown Jinchuuriki Four - Akatsuki has yet to find and seal two Jinchuuriki: Naruto and this unknown Jinchuuriki. It is presently unknown what number of tails it has. Dialogue seems to indicate it may be the Hachibi, however one theoretical model for the King of Hell eye openings indicates it may be the Gobi.
http://www.leafninja.com/akatsuki.php
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